Killed in action (KIA) is a casualty classification generally used by militaries to describe the deaths of their own forces at the hands of hostile forces. The United States Department of Defense, for example, says that those declared KIA need not have fired their weapons but have been killed due to hostile attack. KIAs do not come from incidents such as accidental vehicle crashes and other "non-hostile" events or terrorism. KIA can be applied both to front-line combat troops and to naval, air and support troops. Someone who is killed in action during a particular event is denoted with a † (Unicode U+2020: Dagger) beside their name to signify their death in that event.

Further, KIA denotes one to have been killed in action on the battlefield whereas died of wounds (or DOW) relates to someone who survived to reach a medical treatment facility. The North Atlantic Treaty Organization (NATO) also uses DWRIA, rather than DOW, for "died of wounds received in action." However, historically, militaries and historians have used the former acronym.